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17 degree temperature change... Well I'm guessing that you're suffering from local seeing - you should be able to tell if the mirror is too hot by pointing at a star and defocussing the image. Look for rotating spikes around the edge of the defocussed star. Thats a sure indication of warm air spilling off the side of the mirror. Not sure if that applies to a closed tube SCT though...hmm.
Before I built this peltier cooler I could see that happening all night long. Set up at 8pm, still getting those heat spikes at 3am. Pack up in disgust.
I've seen a "SCT cooler" that was a dongle that you poke up the inside through the eyepiece hole. It has a fan to circulate air. Now I'm thinking that you could bolt a peltier cooler onto it and make it push cooler air into the scope.
Bird
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